In remote areas and areas with no or unreliable mains a photovoltaic array may be utilized for driving a load, such as an electric motor of a water pump or a ventilator.
An optimal operating point of a photovoltaic array is not constant but depends on the solar irradiation intensity, the temperature of the photovoltaic array, and the type of the photovoltaic cells employed in the photovoltaic array. Therefore, a photovoltaic array is frequently controlled to operate at the maximum power point of its I/V characteristic, thus gaining maximum benefit from array investment.
A conventional photovoltaic array output power measurement using controller analogue-digital-converters (ADC) has relatively poor accuracy. In particular, the output power measurement is subjected to inaccuracies and to both integral and differential non-linearities associated with ADCs and subsequent integer arithmetic. This may result in less than optimum performance or in settling at a spurious power peak away from and smaller than the real maximum power peak of the photovoltaic array.
It is known to use Maximum Power Point Tracking (MPPT) with photovoltaic arrays to maximize the power extraction from a photovoltaic array. The central problem addressed by MPPT is that the efficiency of power transfer from a photovoltaic array depends amongst others on the electrical characteristics of the load driven by the photovoltaic array. When, for example, the solar irradiation intensity varies, the load characteristic that gives the highest power transfer efficiency changes, so that the efficiency of the solar drive system is optimized when the load characteristic changes to keep the power transfer at highest efficiency. This load characteristic is called the maximum power point and MPPT is the process of finding this point and keeping the load characteristic there. MPPT solves the problem of choosing the best load to be presented to the photovoltaic array in order to get the most usable power out. It is the purpose of MPPT to sample the output of the photovoltaic array and apply a proper resistance (load) to obtain maximum power for any given environmental conditions.
MPPT devices are typically integrated into an electric power converter system that provides voltage or current conversion, filtering, and regulation for driving various loads, such as electric motors. Solar inverters convert the DC power to AC power and may incorporate MPPT. Such inverters sample the output power (I-V curve) from the photovoltaic array and apply the proper resistance (load) so as to obtain maximum power. Maximum power point trackers may implement different algorithms and switch between them based on the operating conditions of the photovoltaic array.
A special MPPT method is the Perturb and Observe (P&O) MPPT method. In this method a controller adjusts the load by adjusting the voltage of the photovoltaic array by a small amount in a step direction and measures the output power of the photovoltaic array. If the power increases, further adjustments in that step direction are tried until power no longer increases. If the power decreases, the step direction is changed.